Centrifuges of the type concerned are called "screen bowl" centrifuges to distinguish them from centrifuges of the same type but without a screen section called "solid bowl" centrifuges. In some screen bowl centrifuges of the prior art, the screen section has formed all or nearly all of the bowl. In others, the screen section follows a solid bowl section in which the primary separation of solids from liquid takes place, the screening section serving to drain and dry the solids preliminary to discharge. While the invention is generally applicable to screen bowl centrifuges, it is described and illustrated herein as applied to such centrifuges of the latter form.
The screen section of screen bowl centrifuges has been typically formed as a lattice of crossing axial and radial ribs defining spaced openings in which screen inserts are secured. The ribs and screen inserts form an inner surface of substantially constant radius from the bowl axis, over which the solids are moved toward an outlet by the helical conveyor. An example of such a screen bowl centrifuge is disclosed in U.S. Pat. No. 3,401,800, where in the screen inserts have a screen portion of which the inner surface is curved at the radius of the inner surface of the cross-rib lattice and a support frame portion with side and end flanges which overlap the outer surface of the sides and ends of the opening in which the insert is placed and are secured thereto. The screen portion is formed of parallel bars secured at their ends to the frame and closely spaced to provide narrow screening slots, the ribs and slots being arranged to extend transversely to the centrifuge axis, that is, circumferentially of the bowl and bridging the opening.
Screen bowl centrifuges in accordance with said patent disclosure have had extensive commercial use despite certain shortcomings, such as rapid wearing away and breakage of the screen bars and plugging of the screen, particularly when exposed to hard, abrasive solids, as, for example, in the dewatering of coal fines. The wearing, breakage, and plugging, is, e.g., caused by wedging particles between the conveyor blade and the screen. To alleviate the plugging problem, the screen bars have been formed of tapered cross-section, so that the slots enlarge outwardly as shown in the aforesaid patent. However, this aggravates the wear problem, since the narrowest, correctly sized portion is of nearly zero depth and can be quickly worn away to an unacceptably large slot width while plugging by hard particles is not materially inhibited.
Efforts have been made to mitigate the wear and breakage problem in commercial screen bowls by the use of hard, highly wear resistant materials, such as chromium surfacing on stainless steel screen bars, or forming the bars of ceramic material or sintered tungsten carbide. The results of such efforts up to now have been disappointing. While the hard material improved wear resistance so long as it remained intact, breakage occurred too soon and too extensively. In the coated bars, cracking or breakage occurred in places where hard trapped particles were forced against it, such cracking or breaking resulting in stripping of the coating from large contiguous areas. The hard material of the bars was prone to break apart, usually near their ends. Since it was thought that pressure across bars circumferentially arranged might be the primary cause of breakage, the hard, wear resistant bars have been rearranged in some commercial machines so that they are parallel rather than normal to the bowl axis, but this has not materially helped the problem either with bars formed of, or coated with, wear resistant materials.